The present invention relates in general to automatic control of fresh or recirculated air flow into a vehicular heating, ventilating, and air conditioning (HVAC) system. More specifically, the invention relates to controlling recirculated air flow to improve efficiency for an internal combustion (IC) vehicle having start/stop capability by increasing IC engine off time.
Fuel economy of automobiles is an important attribute of vehicle performance which is determined by the technologies employed in the vehicle design, by driver behavior and actions, and by conditions under which the vehicle is used (e.g., speed, road design, weather, and traffic). Manufacturers continuously strive to deliver better fuel economy. One technology being increasingly used is known as automatic Start-Stop technology, wherein an internal combustion engine automatically shuts down when the vehicle comes to a stop or coasts and then restarts as needed to continue driving. The reduction in the amount of time the engine spends idling (e.g., while waiting at a traffic light) results in improved fuel economy and reduced emissions. According to some estimates, start-stop technology can provide a 5% to 10% improvement in fuel economy or more.
In addition to vehicle propulsion, the combustion engine drives other vehicle systems such as an air conditioning compressor. Occupant comfort must be maintained during the time that the engine is stopped. Since the air conditioning compressor typically runs on a front-end-accessory-drive (FEAD) belt driven by the engine, the conventional compressor does not run when the engine is stopped. Thus, when the air conditioning system is actively being used and the engine stops during an idle condition, the cooling action is interrupted and the passenger cabin may become warmer. If the cabin temperature increases by a certain amount, the engine is usually restarted so that cooling resumes, but some of the fuel economy improvement may be lost. One example of a strategy for controlling the engine off time is provided in commonly assigned, co-pending application U.S. Ser. No. 13/561,328, filed Jul. 30, 2012, entitled “Engine Start-Stop Control Strategy for Optimization of Cabin Comfort and Fuel Economy,” which is incorporated herein by reference in its entirety.
In attempting to lengthen the time span until it becomes necessary to resume operation of the air conditioning system, the use of cold storage systems has been considered. In one type of cold storage system, an evaporator may incorporate a phase change material that gives off heat (e.g., freezes) during normal operation before a stop event and then absorbs heat by changing back to a liquid phase during the stop event. However, cold storage devices are expensive, are difficult to package due to their larger size, and require additional controls. Moreover, since they consume additional energy during engine operation, the fuel economy improvement is lessened.
Another approach for providing air conditioning while the combustion engine is off involves the use of an electric compressor running off stored electrical energy from a battery. In a typical gasoline-powered vehicle, however, the expense of such an auxiliary air conditioning system is usually prohibitive. Even in a hybrid vehicle (i.e., having both a combustion engine and an electric propulsion system), the additional use of the electric compressor would result in the loss of fuel economy. Thus, it would be desirable to maintain passenger comfort with longer engine off times without relying on cold storage or backup cooling systems.
When heating and cooling systems were first introduced, incoming fresh air was relied upon for both heating and cooling. As systems developed, a recirculation mode was introduced in which cabin air is recycled through the HVAC system since it will already have a temperature closer to the desired temperature than the outside air. Besides full recirculation, a partial recirculation mode may also be used in which an inlet mechanism adjusts a proportion of fresh air to recirculated air that is inlet to the HVAC system via the HVAC blower.
A system and method for a partial air inlet control strategy is disclosed in U.S. Patent Application Publication 2012/0009859A1, which is incorporated herein by reference. It discloses that if the air entering the HVAC is not managed carefully, fuel economy and battery consumption may not be optimized. Particularly, if the fresh air mode is selected as the source of air for the HVAC system in hot weather, this air mode will add more cooling load to the compressor and increase energy consumption. On the other hand, if the fresh air mode is selected as the source of air for the HVAC system in cold weather, this air mode will slow down heater/defrost performance. A further complication is that when the full recirculation mode is selected, window fogging may result in certain ambient conditions. Thus, partial recirculation control strategies have been developed in which the air inlet door is controlled to move progressively to partial recirculation positions by taking into account the cooling/heating loads and the probability of fogging. As cooling/heating loads increase, the air inlet door moves toward a 100% recirculation mode. As fogging probability increases, the air inlet door moves toward a 100% fresh air mode. By selectively choosing a position between 100% recirculation and 100% fresh air, fuel economy and/or battery power consumption are optimized without compromising passenger comfort or causing fogging on interior glass surfaces.
In connection with a start-stop engine, a recirculation setting with a full or partial contribution of fresh air would cause a faster warming of the air conditioning evaporator core than would occur at a full recirculation setting. However, it would be undesirable to allow any fogging of the windows to occur during an auto stop event of the engine. Therefore, the same recirculation strategy has remained in effect during stop events.